1. Field of the Invention
The present invention relates to a method for closed loop control of an optical link. The method provides a novel approach to the control of such a system through the use of a copper feedback link between the optical receiver and optical transmitter to directly define the laser diode threshold bias and modulation levels that are optimum for the link for a defined bit error rate, compensating for link losses and variations with respect to temperature, manufacture, aging, and other factors.
2. Description of Related Art
A typical optical link according to the prior art includes a laser driver, a laser diode (such as a fabry-perot diode or a VCSEL), an optical fiber, a photo diode and a trans-impedance amplifier. The laser driver switches current through the laser diode, which results in an optical emission from the laser diode. By aligning the laser diode and optical fiber, the laser diode emission is then channeled through the optical fiber, although some of the emission will be lost due to manufacturing limitations in the alignment of the laser diode and the optical fiber. At the other end of the fiber the photo diode receives the optical emission (with similar losses due to alignment here as well), which results in current flow through the photo diode. The photo diode is connected to the input of the trans-impedance amplifier, and the input current is realized back into a voltage signal at the output. Optical emission is generally referred to in units of power.
In addition to the alignment issues, optical links are subject to a variety of other factors that cause the emission power to vary, such as temperature, aging of the laser diode, etc. In the prior art, optical links have generally been used to span significant distances, on the order or meters or kilometers. Thus, the receiver and transmitter in an optical link have typically been connected only through an optical fiber that is used solely for forward communication. While there have been various attempts to compensate for some of the factors that cause variations in emission power in the design of the transmitter or receiver, feedback has generally been limited to a loop around only the transmitter, and not the entire system.
In one typical approach, the local mean emission power of the transmitter is measured with an external resistor or a monitor diode that is coupled to the laser diode. If the mean power is too low, the voltage to the laser diode is increased. This approach provides some compensation across both temperature and time-based degradation of the laser diode, but requires extra manufacturing costs to provide the resistor or monitor diode. Since this approach is not able to determine the exact threshold voltage of the laser diode, only the mean power may be used. Further, a margin must be added to the mean power measured to insure that the voltage provided to the laser diode is above the diode's threshold voltage, which tends to increase as the diode ages. In addition, the use of a resistor, or the coupling of the monitor diode to the laser diode, does not accurately represent the alignment of the laser diode and the optical fiber, and thus cannot well compensate for limitations in the accuracy of that alignment.
Alternatively, complete open loop control of the laser diode may be implemented. In one approach, analog techniques are used to model the laser diode characteristics with respect to temperature and current bias. However, this approach offers limited precision and is usually tuned to a specific laser diode, and thus impacts both overall performance and flexibility. Additionally, there is no compensation in this approach for aging of the laser diode.
Complete open loop control of the laser diode may also be implemented digitally by the use of a memory. The memory may be pre-programmed with a generic characteristic, or may be optimized during production on an individual unit basis. Pre-programming requires the attendant silicon and memory costs as well as sub-optimum performance due to diode manufacturing tolerances. Optimizing the memory results in improved performance, but at the expense of costly and complex programming requirements on the production line.
The receiver may also be designed in a particular way to compensate for some of the described problems. For example, the receiver may be designed to support a system-specified bit error rate criterion for its incoming signal. However, this requires allowing for both the smallest and largest possible current signal given the manufacturing tolerances, variations and operating conditions of the system, if such concerns are not dealt with by compensation techniques within the transmitter. In this approach, the constraints on a transmitter may be relaxed; for example, the transmitter may be allowed to have a larger variation in extinction ratio with temperature. But this relaxation comes at the cost of increasing receiver complexity, in this case increasing the receiver's operating dynamic range. Requiring operation of a receiver over a wide dynamic range is a significant design challenge, and greatly increases design complexity.
None of these approaches specifically compensate for losses due to alignment problems. To the extent that prior art solutions attempt to compensate for link loss due to alignment difficulties, they do so by fixed margins, representing an additional fixed power overhead.
Finally, optical links are becoming more common in small consumer devices such as cell phones, PDAs, etc. These devices present two particular problems. First, as it is desirable to keep the cost of the devices down, low cost, and thus low performance, components are typically used. To maximize the life span of these components, it is preferable that they be driven at the lowest levels possible, as hard driving of the components (at or near their upper limits, for example) will often shorten their life span. For example, the laser diode should be driven at a level as close to its threshold voltage as possible. Second, to extend the operating time of the devices per battery charge, it is preferable to keep the operating power to the minimum possible.
In the absence of any feedback between the receiver and transmitter certain challenges are thus presented in the design of the elements in an optical link system. It is therefore desirable to have a method of controlling an optical link that compensates for most or all of these issues and supports optimum power performance while requiring low operating power levels.